Tuesday, December 24, 2013

Dynamic Tethers

 rev. 4-19-2014

Stitched Splice Warning. Based on limited testing we have found that nylon rope is much different to sew than polyester rope. Because of the extreme elongation prior to break, the stitching at the tail must carry nearly all of the load, while the stitching near the throat carries no load. It is rather like tug-of-war with a bungee cord; only the 2 men nearest the center can do any work.

Because of this difficulty, I strongly suggest all dynamic tethers be knotted rather than sewn. Climbing ropes have very high knot strength and are drop-tested with a figure-8 to attach the mass. A double overhand noose has also been drop tested.

This subject is a little too esoteric--for most sailors--and so magazines aren't much interested.   Fatalities due to tethers breaking are rare. On the other hand, bruised ribs and back injuries are more common, but wrongly accepted as a part of rough weather sailing. However, if  we simply apply what we know from climbing about falling--lots or real world experience and lots of lab testing--it just doesn't need to be that way. Let's see if I can sweep away a few bits of conventional wisdom that are just plain wrong. Though you may never sail the Sydney/Hobart, capsize your boat and test the limits of  the human body, routine bumps against tethers don't need to hurt either. We can make our sailing experience more comfortable and safer at the same time. My tethers don't hurt.

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I started my investigation into soft tethers and impact forces years ago, but honestly, my boat is not the ideal test bed; catamarans don't heel, I don't sail in big waves, and I'm not as willing to risk physical trauma as I used to be. I hold on and I don't fall. However, I've let myself slam into tethers intentionally a few times, just for testing, and I'm not happy with the current state of design. My back is too old.

Let's step back for a moment, before we charge forward, and state the limits of our investigation. We're not discussing jackline and energy absorption. I've covered that before and common experience indicates that a jackline will absorb enough energy to safely slow the sailor. I am discussing only the case of a sailor clipped to a rigid point.

While the Screamer/webbing combination is a huge safety improvement over the typical non-elastic tether, it is really engineered for a different purpose; to catch severe impacts in a survivable way for climbers and workers wearing hip or full body harnesses. Sailors, on the other hand, have a chest harness, which is lousy at distributing force. The screamer is just too hard.


A little backyard testing

I do NOT suggest this as you can easily hurt your spine. I've climbed enough to know what I was getting into.
  • Anchor one end of the tether to an immovable object, in this case a large tree wrapped with non-stretch line.
  • Create several tethers, 6 feet long from test materials: 1-inch nylon webbing,  1-inch webbing with Yates screamer, 10.4 mm dynamic rope (UIAA single), 8.2 mm dynamic rope (UIAA double).
  • With increasing amounts of slack in the line, run at the end of the tether as hard as you can back first.
Results:

1-nylon with Screamer. A very tough stop. With about 3 feet of slack I was able to just get the Screamer to trigger and it hurt. My back will complain for a few days.
10.4 mm rope. Firm stop. I didn't go further than 4 feet, not wanting injury. I'm sure I could have run from 6 feet with no harm, though it would be a jolt.
8.2 mm rope. Even at 6 feet and with a sore back, the stop was practically fun. It might take you off your feet, but was no harder than hitting a well padded couch. At 2-3 feet it was only amusing, with about 1-foot of stretch.
1/4-inch Amsteel. I was smart enough not to try. It would be worse than webbing without a Screamer.

General Observation. I wear my harness high, nearly in the armpits and definitely on my lats. If the harness had been worn lower, where catalogs show them or where combo PFDs fit, I would have injured my ribs.  Whiplash is also a problem. Wearing a harness down near the solar plexus and lower ribs, where so many are located, is plainly dangerous.

In the end, only the 8.2 mm rope felt good, thought the 10.4 was livable.


Data and Calculation

Is it strong enough? 8.2 mm line is not 5000-pound rated, but as we will see, it is far tougher (can absorb more energy) than 1-inch webbing.

What do we know from laboratory and real-world fall data? For new products....
  • Webbing tethers are just adequate: They occasionally fail at forces near 4000 pounds. Rare, but we can use that as a design point. 
  • 1-inch webbing can just manage a 6-foot fall with a 185-pound rigid mass; this is generally considered as equivalent to a 200-pound person, since the harness and body absorb some energy by deforming (data from DMM, a climbing equipment manufacturer).
  • 10.4 mm UIAA single rope can manage more than 10 falls from nearly twice this distance.
  • 8.2 mm UIAA double rope can manage more than 10 falls from nearly twice this distance with a 242 pound rigid mass (261-pound person) using 2 strands.
  • 8.2 mm UIAA double rope can manage more than 10 falls from 6 feet with a 242 pound rigid mass (261-pound person) using 1 strand. Thus, it is more than 50% tougher than 1-inch webbing.
  • 8.2 mm UIAA double rope can manage one fall from nearly twice this distance feet with a 242 pound rigid mass (261-pound person) using 1 strand. Again, it is more than 50% tougher than 1-inch webbing.
  • 1/4-inch Amsteel is the worst material that could be used. While strong, its energy absorption capability is negligable. Its only advantage over chain or cable is light weight.
I created data using well-used products. Why used? Because they will become so, obviously. Less documentable and less repeatable by others--science is all about reproducible results, be they real-world or not--but ultimately more practical.
 
 Percent Elongation (left) vs. Pounds Stress, pounds (bottom)

Webbing is 3-5 times stiffer than dynamic rope. Amsteel is very stiff.

I took the samples and pull tested them up to 2500 pounds; I didn't feel like destroying my tether just yet. I used on-line data  to extrapolate out to the breaking point of each material. In dynamic situation, the breaking strength and the elongation to break is typically 30% less and energy absorption about 3 times less, but for blog post purposes, the above data at least allows comparison on an equal basis.

What matters is the area under the curve.

Static Energy Absorption, 6-foot tether, ft-pounds (left) vs. Stress, pounds (bottom) 

Static calculated energy values are greater than fall values for 3 reasons:
  • Ropes are tested with figure-8 knot, weakening rope by ~ 40%.
  • Dynamic testing does not allow for heat dissipation and reorientation of fibers.
  • Dynamic ropes survive 8-12 falls, the webbing only 1 fall in our test data. Dyneema survived zero test falls.
Dynamic Energy Absorption, 6-tether, ft-pounds (left) vs. Stress, pounds (bottom) 


 At lower stress, 8.2 mm rope absorbs more energy for a given impact force, but eventually 10.4 mm rope has greater capacity. Both dynamic ropes have greater energy storage capacity than webbing. 8.2 mm rope is more than twice as tough as 1-inch webbing.

Both this data and UIAA drop data thus confirm the following:
  • 11mm rope can absorb many fall factor 2 drops.
  • 8.2 mm rope can absorb 1 fall factor 2 drop.
  • 1" webbing can absorb 1 fall factor 1 drop.
What happens in a 6-foot test fall?
  • 1-inch webbing: At 4000 pounds impact, 1100 ft-pounds energy. We know from fall testing, however, that 1-inch webbing can only absorb about 1100 ft-pounds (sometimes passes, sometimes fails), so we will use 1100 ft-pounds as the design point.
  • 10.4 mm rope:  If we accept the design point only 1100 ft-pounds of energy, impact force is about 1400 pounds. New rope fall data indicated significantly less.
  • 8.2 mm rope: If we accept the design point only 1100 ft-pounds of energy, impact force is about 1100 pounds. New rope fall data indicated significantly less.
  • 1/4-inch Amsteel: Only 236 ft-pounds of energy were absorbed by the point 5000 pounds of stress were reached, and only 677 ft-pounds of energy were absorbed at the break strength of 8600 pounds. Amsteel will kill the sailor and pull the harness apart before surviving a 6-foot fall. This has been duplicated in real-world fall testing with spectra products; Dyneema is worse.
If we limit the fall to something more reasonable--a 2-foot fall is similar to wave strike and 6 knot stumble, and more severe than my backyard testing--the energy for me is only165# x 2' = 330 ft-pounds, which can be absorbed by a 500-pound impact; you'll feel it but not get bruises. With a webbing lanyard it would be 3 times that pressure and you will be injured. A Screamer will trigger but not be fully spent (though they are single-use).

How much energy can a Screamer absorb? About 270 foot-pounds according to several testing sources, or about 25% of a 6-foot fall; in combination with the stretch of the webbing, enough to reduce the impact force below 2000 pounds. And then you bottom out and the force goes up, though less so.

Other Factors. Does the extra stretch bother you? Probably personal. Is stepping on rope a greater hazard than webbing? For 8.2 mm rope, not much, but on a monohull (sloping decks) that may be more of an issue. Construction? Either knots (testing assumed figure-8 knots) or sewn eyes are practical.

A critical biometric factor, not easily relayed by a machine or simple data, is that a force that comes on gradually (the dynamic rope) allows the muscles time to tighten and resist. A sudden 500-pound kick before a screamer gives is painful and damaging, like a blindside blow, even at the same force level. You truly feel the difference. 

Dynamic rope or a screamer--The best solution? For smaller, more routine falls on the deck, the rope is kinder. For that once in 100 sailors' lifetimes, torn from the cockpit by a rogue wave impact, the screamer/webbing combination is competitive, though the climbing rope tethers are still survivable without serious harness impact injury; both could have impact forces of 1000-1500 pounds depending on the force of the wave, and it would hurt, but not break ribs or vertebrate. A climbing rope, even 8.2 mm, has greater energy absorption capacity than a webbing/Screamer combination. Thus few climbers use Screamers, but they ALL use dynamic ropes.

The only disadvantage of rope over webbing is the  potential for rolling under foot. A dynamic webbing would be nice, but I could not find any. On a catamaran this is a limited problem (level decks), and the roll-under-foot potential is reduced by the small size of 8.2 mm line (5/16-inch).

What have I done? I've switched to 8.2 mm dynamic tethers. It is kinder on the back for minor falls and even over the edge. It is lighter than the webbing/Screamer combination and easier to hold on to.

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Where to get limited quantities of dynamic (climbing) rope?
MEC sells both single and half rope off spools, by the foot.
Half Rope,
Single Rope.

2 comments:

  1. Have you considered testing a via ferrata tether?

    ReplyDelete
  2. Actually, I have (I have a lot of climbing gear). I will post on the topic in a few days. There are commonalities and differences.

    ReplyDelete